In the semiconductor industry, there is a continuing trend to increase device density by scaling device size. In typical state of the art devices, device dimensions are in the order of one micron, or slightly below. As the trend towards smaller, more densely packed devices continues, critical dimensions of future devices can be expected to be in the range of approximately 0.3 through 0.5 microns or smaller.
In order to achieve these reduced dimensions, improvement in lithographic performance is necessary. As is well known, the resolution (minimum feature size which can be formed in a photosensitive layer) is proportional to .lambda./NA, where .lambda. is the wavelength of the exposing radiation, and NA is the Numerical Aperture of the projection lens of the lithographic system. Therefore, in order to improve the resolution, systems using a smaller wavelength radiation (such as deep UV or X-ray, for example), and/or a larger numerical aperture, have been investigated.
In a projection lithography system, an image of the pattern on a mask or reticle (the term "mask" will be used herein to encompass any type of substrate having a pattern to be transferred) is formed by focusing the radiation exiting the mask through a lens onto the photoresist layer. The image is formed at a focal plane within the photoresist layer. This focal plane is called the plane of "best focus." Additionally, a satisfactory image is formed for a certain distance (defocus) away from the best focus. The depth of focus (DOF) is a measure, along the light axis, of this distance over which the image has sufficient contrast and intensity to define a pattern in the photoresist layer. While the depth of focus will vary based upon the lithography system and exposure parameters used, a satisfactory image can typically be formed at approximately .+-.2 microns defocus for features in the 1 micron range. Therefore, any portion of the photoresist layer within the depth of focus can successfully be patterned, while any portion of the photoresist which is either above or below the depth of focus will not be patterned, due either to poor contrast or insufficient exposure intensity.
The depth of focus is proportional to .lambda./NA.sup.2. Therefore, the above described attempts to improve resolution by decreasing .lambda., or, to a greater extent, by increasing NA, will result in a reduced or shallower depth of focus. Thus, as feature size becomes smaller, depth of focus becomes a more important consideration. The effect of the depth of focus is discussed in relation to FIG. 1. FIG. 1 shows a semiconductor substrate 100, having for example metal layer 101 formed thereon and a line and space pattern in the photoresist comprising lines 102 through 106 with spaces 107 through 109 therebetween. Typically, the thickness of the resist layer is approximately 1 micron, and a satisfactory image must be maintained throughout this thickness in order to form a pattern. When the feature size is approximately in the range of 1 micron or larger (i.e., width of lines 102-106 and spaces 107-109) a printer and exposure parameters can be used which give sufficient depth of focus such that the image quality at the top level 115 and bottom level 120 has sufficient intensity and contrast. Typically, the image is focused at approximately the center of the unpatterned photoresist layer, i.e., approximately mid-way between levels 115 and 120. As mentioned above, as resolution is improved, particularly by increasing NA, the depth of focus is reduced. In a system to image photoresist wherein the dimensions of the lines and spaces are, for example, approximately 0.5 micron or below, the system will not have sufficient depth of focus to pattern the image at all points along the light axis within the photoresist. This problem is further compounded by the fact that there may be some lines and spaces, such as lines 105 and 106, and space 109, which are at a higher or lower level than other lines and spaces, due to the varying topography of the wafer. Additionally, other factors, such as warpage of the wafer, non-planarity of the stage in the lithography system, etc., may further increase the distance between the lowest level and the highest level of the photoresist layer to be patterned.
One solution to this problem, called the focus latitude enhancement exposure (FLEX) has been proposed to increase the focus latitude in projection lithography. In this method, the substrate is exposed to radiation multiple times, at different levels along the light axis. In the FLEX method, the substrate is first exposed with the pattern focused such that its focal plane is at, for example, the level 115. Next, the substrate is moved relative to the mask and lens, either by moving the substrate stage up or moving the mask/lens assembly down, and a second exposure is performed with the focal plane at the level indicated by, for example, level 120. The difference in distance between the focal planes of each of the exposures, .DELTA.F, is adjusted so that exposure intensity along the light axis is sufficient to expose the entire photoresist layer, regardless of the above described variations in topography.
FIG. 2 shows plots of intensity at various levels along the light axis 200. In the column 201, the exposure intensity along the light axis 200 is shown for a conventional, single exposure system. As shown, a well defined intensity peak is present at zero defocus. At .+-.1 micron along the x-axis, the intensity of the radiation has diminished but a strong peak is still present. At .+-.2 microns, the intensity of the exposing radiation is greatly diminished. Thus, the depth of focus extends from approximately -2 through +2 microns, giving a depth of focus of approximately 4 microns. With the FLEX method as described, two exposures, for example, at approximately -2 microns as shown in column 202 and approximately +2 micron as shown in column 203 would be performed. The combined exposure along the light axis would be as shown in column 204 which adds the exposure in columns 202 and 203. As can be seen, the depth of focus has been greatly extended, to approximately -3 through +3 microns. Further improvement in depth of focus can be achieved by performing more than two exposures. In the FLEX method, an important parameter is the distance between focal planes, .DELTA.F. If .DELTA.F is too small (focal planes close together), then only minimal improvement in depth of focus will be achieved. Furthermore, a small .DELTA.F together with a large number of exposures spaced by .DELTA.F cannot be used, because the light intensity will become relatively flat (poor contrast) at any given level due to the presence of radiation from the numerous defocused images. This is particularly true for patterns where interference from neighboring patterns in present, such as closely spaced line and space patterns. On the other hand, if .DELTA.F is too large, then the regions between the focal planes will not be exposed. Therefore, .DELTA.F must carefully be optimized for the features being formed and the printer and exposure parameters being used, to give acceptable intensity and contrast throughout the extended depth of focus.
While the above described FLEX method increases the depth of focus, one drawback is that the multiple exposures per field which are required greatly increases process throughput time. Additionally, since plural exposures are used for single pattern, any misalignment will result in image degradation. If an alignment step is performed for each exposure, this will further increase the processes throughput time.
What is needed is a method and system for enhancing the focus latitude in lithographic systems which does not increase throughput time and preferably decreases the throughput time of the exposure process.